When you sell a share of ownership in a business, the capital gains tax rates a short term if you sell it within the first year of acquisition, and long term afterwards.
If you sell a share of ownership in a business you founded (as opposed to something you bought on a stock exchange, for example), the date of acquisition is generally when you started investing in the business. You may have initially started it as a sole proprietorship and then reorganized in some other way, but your basis carries through all these stages. Incorporating as a C-Corp may break that flow as you create an entity separate from yourself for tax purposes, and as such your basis in the corporation carries, but the date of acquisition of the corporation is when it was created.
For more complicated business with large balance sheet and a lot of capital infusions at different time points you may need to work with an accountant to calculate your basis and your gains.